The idea of using polymers as supports for catalysts has been known since the late 1950's and the polymers used as supports have mainly been crosslinked polystyrene or polyvinylpyridine based materials. Reagents supported on crosslinked polymer matrices have many advantages compared with low-molecular weight species, like ease of separation, reuse of catalysts, adaptability to continuous flow processes, reduced toxicity and odor. The main disadvantages with these crosslinked reagents are higher cost, lower reactivity due to diffusional limitations and greater difficulty of analysis of the supported species and impurities. Furthermore, they are mostly in the form of powders which always have to be filtered from the reaction media (Hodge, P., Sherrington, D. C., (Eds.), Synthesis and separations using functional polymers, John Wiley & Sons, New York, (1988); Ford, W. T., (Ed.), Polymeric reagents and catalysts, American Chemical Society, Washington, DC, (1986)).
A polymer support used in a particular application should fulfill a number of important functions simultaneously. First, the support must possess the correct mechanical properties. For example, in column or batch applications, supports must be mechanically strong to resist compression and fraction into fine particles. Secondly, the support must possess the correct physical structure in order to ensure that a high amount of the functional groups in the material are accessible to the reaction phase. Finally, the support must provide the correct microenvironment to optimize the process being carried out, e.g. it must provide the correct polarity, hydrophilicity, microviscosity etc. In general, these support requirements have been demanded by default rather than by careful argument and design (Guyot, A., Reactive Polymers. 16, (1992), 233).
A useful method for preparing polymer bound reactants, with a potential of solving many of the problems mentioned above, is grafting and, especially, radiation grafting offers promising new opportunities (Hartley, F. R., J. Polym. Sci., Polym. Chem. Ed., 20, (1982), 2395; Garnett, J. L., J. Polym. Sci., Polym. Lett., 19, (1981), 23; Akelah, A., J. Appl. Polym. Sci., 28, (1983), 3137). This method involves taking a polymer with appropriate morphology and physical properties and introducing reactive sites, free radicals, into the polymer chain by irradiation. The free radicals can either combine to give cross-links, as is the case of, for example, polyethylene, or cause chain scission, as in the case of, for example, polypropylene. In the presence of vinyl monomers, on the other hand, the free radicals can initiate graft copolymerization.
The preparation of graft copolymers and the use of graft copolymers in a variety of different applications are well known both in the scientific literature and patents (Stannet, V. et al., Radiat. Phys. Chem., 35, (1990)).
Three different methods of radiation grafting have been developed and most of the work has concentrated on the use of low dose rate gamma rays from .sup.60 Co sources. During the past few years, however, there has been much interest in using high energy electrons from accelerators with high dose rates (10.sup.6 -10.sup.9 rads/sec), since these high dose rates make radiation chemical processes commercially more attractive. The chemistry involved is, however, similar whether gamma or electron radiation is utilized, and therefore the graft resulting from different sources does not significantly differ. The three methods of radiation grafting that have received special attention are: (1) direct radiation grafting of a vinyl monomer onto a polymer (mutual grafting), (2) grafting on radiation-peroxidized polymers (peroxide grafting) and (3) grafting initiated by trapped radicals (pre-irradiation grafting).
The mutual grafting by irradiating the polymer in the presence of the monomer is a fairly simple and effective method, since the free radicals initiate polymerization immediately as they are generated. The disadvantage of this method is, however, that simultaneously with the graft copolymerization, homopolymerization of the monomer occurs upon irradiation.
When grafting radiation-peroxidized polymers, the polymer is first irradiated in the presence of oxygen, thus forming peroxides and hydroperoxides that are stable and can be stored in the polymer for a long period of time. Grafting is activated by cleavage of the peroxides or hydroperoxides by heat, UV-light or catalysts in the monomer solution.
The pre-irradiation grafting by irradiation of the polymer alone in an inert atmosphere and immersing the irradiated polymer in a monomer solution requires additional steps in comparision to direct grafting, but the advantage is that only a small amount of homopolymer is formed, mainly by a chain transfer process. The grafting process is controlled by the diffusion of the monomer into the polymer and can to some extent be facilitated by the use of solvents that are able to swell the formed graft copolymer.
Pre-irradiation grafting is mostly preferred since this method produces only small amounts of homopolymer in comparison to mutual grafting.